This invention relates to optical fibre gratings.
To allow a high rate of data transmission through existing fibre links which are inherently dispersive, it is necessary to provide some means for dispersion compensation, such as chirped fibre gratings.
Chirped gratings are gratings in which the Bragg wavelength varies along the length of the grating, in contrast to conventional non-chirped gratings which are characterised by a single resonant wavelength (the Bragg wavelength) defined by a single grating pitch. The fact that a chirped grating reflects light of different wavelengths at different positions along its length can be exploited to provide a dispersion compensator.
For dispersion compensation of wavelength division multiplexed (WDM) signals, multi-channel chirped gratings can be used. Multi-channel chirped gratings have been realised using sampled gratings (see Ouellette et al, Electronics Letters, Volume 31, pages 899-900 (1995)). Sampled waveguide gratings are gratings in which one or both of the amplitude and phase of the refractive index modulation is itself modulated periodically in a binary (two-state) manner along the structure. In use, such a grating provides multiple reflection peaks separated by a wavelength defined by the amplitude and/or phase modulation, the multiple reflection peaks being distributed around a central Bragg wavelength of the sampled waveguide grating.
The individual channels of a multi-channel grating may be chirped to provide a multi-channel dispersion compensator. Multi-channel gratings also have other applications, for example when not chirped as optical filters and pulse shapers of WDM signals.
One kind of multi-channel grating is a Moire grating which can be considered as a superposition of two Bragg gratings of different pitch. Moire fibre gratings have been realised as reported by Reid et al in Electronics Letters, Volume 26, pages 10-12 (1990) and by Legoubin et al in Electronics Letters, Volume 27, pages 1945-1946 (1991).
Reid et al report a single-mode fibre which is lapped and polished to gain access to the field in the core and then coated with a thin layer of photoresist. A Moire grating is formed in the photoresist by a double exposure to interference patterns of slightly different period. After developing the photo-resist, the grating is etched and then coated with a layer of aluminium oxide and finally covered with an index-matching oil.
Legoubin et al report writing a Moire grating into a germanosilicate (Ge/Si) fibre using double-exposure by ultra-violet fringe patterns. An alternative reported technique is to etch a Moire pattern into a phase mask substrate, so providing a bespoke phase mask for that Moire pattern.
The distinction between the channels in these previously-realised Moire fibre gratings lacks clarity, and chirping of the structure has not been reported or even considered possible.